std.mem.indexOfSentinel() vectorized path violates pointer aliasing rules

[alexrp]

zig/lib/std/mem.zig

Lines 1097 to 1142 in 8e0a4ca

	// The below branch assumes that reading past the end of the buffer is valid, as long
	// as we don't read into a new page. This should be the case for most architectures
	// which use paged memory, however should be confirmed before adding a new arch below.
	.aarch64, .x86, .x86_64 => if (std.simd.suggestVectorLength(T)) |block_len| {
	const page_size = std.heap.page_size_min;
	const block_size = @sizeOf(T) * block_len;
	const Block = @Vector(block_len, T);
	const mask: Block = @splat(sentinel);
	comptime assert(std.heap.page_size_min % @sizeOf(Block) == 0);
	assert(page_size % @sizeOf(Block) == 0);
	// First block may be unaligned
	const start_addr = @intFromPtr(&p[i]);
	const offset_in_page = start_addr & (page_size - 1);
	if (offset_in_page <= page_size - @sizeOf(Block)) {
	// Will not read past the end of a page, full block.
	const block: Block = p[i..][0..block_len].*;
	const matches = block == mask;
	if (@reduce(.Or, matches)) {
	return i + std.simd.firstTrue(matches).?;
	}
	i += @divExact(std.mem.alignForward(usize, start_addr, block_size) - start_addr, @sizeOf(T));
	} else {
	@branchHint(.unlikely);
	// Would read over a page boundary. Per-byte at a time until aligned or found.
	// 0.39% chance this branch is taken for 4K pages at 16b block length.
	//
	// An alternate strategy is to do read a full block (the last in the page) and
	// mask the entries before the pointer.
	while ((@intFromPtr(&p[i]) & (block_size - 1)) != 0) : (i += 1) {
	if (p[i] == sentinel) return i;
	}
	}
	assert(std.mem.isAligned(@intFromPtr(&p[i]), block_size));
	while (true) {
	const block: *const Block = @ptrCast(@alignCast(p[i..][0..block_len]));
	const matches = block.* == mask;
	if (@reduce(.Or, matches)) {
	return i + std.simd.firstTrue(matches).?;
	}
	i += block_len;
	}
	},

I think this is pretty clearly a violation of LLVM's pointer aliasing rules. LLVM could break this code whenever and would be fully justified in doing so.